test: make the image bigger

I got some errors about lack of disk space... 100MB either way
shouldn't matter.

test/test-functions

@@ -126,12 +126,12 @@ install_missing_libraries() {
 create_empty_image() {
     rm -f "$TESTDIR/rootdisk.img"
     # Create the blank file to use as a root filesystem
-    dd if=/dev/null of="$TESTDIR/rootdisk.img" bs=1M seek=200
+    dd if=/dev/null of="$TESTDIR/rootdisk.img" bs=1M seek=300
     LOOPDEV=$(losetup --show -P -f $TESTDIR/rootdisk.img)
     [ -b "$LOOPDEV" ] || return 1
     echo "LOOPDEV=$LOOPDEV" >> $STATEFILE
-    sfdisk -C 6400 -H 2 -S 32 -L "$LOOPDEV" <<EOF
-,3200
+    sfdisk -C 9600 -H 2 -S 32 -L "$LOOPDEV" <<EOF
+,4800
 ,
 EOF